In an optical communication system by means of the bidirectional transmission using the light beams whose wavelength are different, an optical communication module including an optical system having a light-receiving element, light-emitting element and lens is provided to the terminal which transmits and receives optical signal through an optical fiber. In most of the conventional optical communication modules, glass-made lenses are used as an optical element constructing the optical system. However, it is considered that the glass-made lens is replaced with resin-made lens because the glass-made lens is comparatively high in cost. However, an non-negligible refractive index change is generated by the temperature change in a general resin-made lens. For example, it makes difficulty of appropriately forming an image on the end surface of the optical fiber using the light flux emitted from the light-emitting element. Thereby, there is a possibility that the coupling efficiency of the optical fiber is lowered.
On the other hand, diffractive ring-shaped zones are formed on an optical surface of the resin-made light converging lens in the semiconductor laser module written in Patent Document 1. Using the fact that an oscillation wavelength of a light beam from the semiconductor laser changes when a refractive index of the light converging lens changes by a temperature change, the semiconductor laser module conducts a temperature compensation by canceling out a focal point movement following the movement of the oscillation wavelength of the semiconductor laser oscillator to the temperature change and a focal point movement following the refractive index change and thermal expansion or thermal contraction by the temperature change of the resin-made lens.
[Patent Document 1] Tokkaihei No. 11-142696
However, in the semiconductor laser module written in Patent Document 1, the temperature compensation becomes available only when a light beam projected from the light-emitting element is sent to the optical fiber. Herein, an oscillation wavelength of a light beam sent by travering through the optical fiber does not change due to the temperature change occurring at the semiconductor laser module side when the light beam sent from the optical fiber is received by the light-receiving element. Therefore, there is a problem that when the refractive index of the light converging lens changes due to the temperature change, its focal point movement is hard to be suppressed.
Further, it is not preferable that a large number of diffractive ring-shaped zones are formed in the light converging lens as shown in the semiconductor laser module written in Patent Document 1, because the structure is not gotten free from the the coupling efficiency lowering and the light amount lowering affects a large influence on a communication. It is also a problem. A technology in which the diffractive ring-shaped zones are used is widely known conventionally. For example, it is adopted for the spherical aberration correction in the field of the optical pickup apparatus, and its purpose includes achieving compatibility between optical disks with different formats (whose substrate thicknesses are different), the chromatic aberration correction, and the chromatic spherical aberration correction. A focus is finely adjusted by auto-focusing the objective lens, and it is difficult in the design work that the focal point movement by the oscillation shift of the semiconductor laser and the focal point movement by the refractive index change by the temperature change of the resin-made lens and the thermal expansion or thermal contraction are balanced accurately.